Fluence dependence of deuterium trapping in graphite

The trapping of ∼ 1 keV D in graphite at 300 K has been studied by thermal desorption spectroscopy, as a function of fluence, for fluences up to ∼ 4 x 1024 D/m2. In pyrolytic (HPG99) and fine grain (EK98) graphite, the amount of retained deuterium was found to increase with increasing D+ fluence over the entire range studied. In a pseudo-monocrystal sample, however, the amount of retained deuterium did not increase significantly beyond the saturation concentration of ∼ 2 x 1021 D/m2. The difference in retention properties is attributed to the crystal structure of the graphite. The more porous structure of pyrolytic and fine-grain graphites allows the migration of thermalized D atoms beyond the implantation zone, where they may be trapped, adding to the total hydrogen inventory. (orig.)